Ophthalmic lens is defined as a lens suitable for carrying on the eye or inside the eye. Also included are less common vision correction lenses such as artificial corneas and lamellar corneas implants. There is a significant effort to develop a lens for presbyopia correction in a form of refractive or diffractive type lenses.
A fixed single power lens provides good quality of vision but only within the small range of viewing object distances that is usually significantly narrower than the range required for near to distant vision. An improved type of the implant provides a number of powers, so called bifocal or multifocal lens. Reference to bifocal or multifocal terminology is used herein interchangeably. The multifocal ophthalmic lens can provide refractive powers, diffractive powers or a combination of both for required range of vision.
Although refractive lenses were first to be developed they may be interpreted as a specific state of diffractive optics and it may be more appropriate to address a diffractive optic definition in order to describe refractive and diffractive surface types. A diffractive lens that follow the specific rule hereinbelow described. If step sizes are zero or are randomly sized or groove areas are also randomly sized, the lens becomes a refractive type, i.e. the corresponding image locations are defined by Snell's law.
A diffraction lens can be considered as a combination of refractive lens formed by zero step size and phase grating, see FIG. 1. A phase grating can be formed by different types of zone or groove shapes where the blaze shape shown on the FIG. 1 is the most common one. Thus, a blaze shape is cut into a base refractive surface to introduce a phase grating, i.e. a periodic array of optical scattering regions.
Scattering light in all directions by the periodic structure creates constructive and destructive interference of light at different but specific angles depending on wavelength of light which are called diffraction orders. The corresponding wavelength of light used to design the phase grating is called design wavelength.
The directions of the orders and corresponding image locations are defined by the Grating formula, not Snell's law. Zero-order diffractive power coincides with the power of the refractive surface formed by the base curvature and, therefore, loosely called refractive power of the diffractive lens. The key point for the grating to perform, i.e. to form distinct diffraction orders, is to have equal areas of Fresnel zones (grooves) and equal Optical Path Differences between adjacent zones at their borders (OPDb) in the direction of each diffraction order.
According to the wave nature of light, constructive interference of light from different grating regions occurs if light is in phase at the corresponding image plane. The constructive interference would maintain if the light from one of the regions is shifted by the full phase equaled to integer number of the design wavelength. For instance, zero order corresponds to the original direction of the light produced by the refractive base curve, i.e. zero phase shift between light coming from each adjacent blaze zone, 1st order is produced by the phase of one wavelength shift between each adjacent blaze, 2nd order is produced by the phase of two wavelengths shift between each adjacent blaze and so on. Grating period or blaze zone spacing determines an angle of the given diffractive order, i.e. the corresponding focal length or diffractive power of the given diffraction order.
By the definition of the diffraction order, light can only be channeled along the diffraction orders of the diffractive lens, i.e. discrete channels, but the percent of totally available light that is actually channeled for a given diffraction order depends upon the light phase shift introduced by each blaze zone, i.e blaze material thickness (h), see FIG. 1. The percent of total light at a given order is called diffraction efficiency of this order. In general terms one can call it a light transmittance for the given order.
According to the “geometrical model” of the grating 100% efficiency (light transmittance) in m-order can be achieved if the direction of the blaze ray defined by the refraction at the blaze coincides with the direction of m-order diffraction, (Carmiña Londoño and Peter P. Clack, Modeling diffraction efficiency effects when designing hybrid diffractive lens systems, Appl. Opt. 31, 2248–2252 (1992)). It simply means that blaze material thickness is selected to direct the blaze ray along the m-order diffraction produced by the blaze zone spacing for the design wavelength of light.
The “geometrical model” provides a simple explanation of the diffractive lens structure which is important in a description of the present invention instead of relying on the mathematics of phase function, transmission function and its Fourier series to calculate diffraction efficiencies and solving the diffraction integral for intensity distribution.
For instance, if the blaze ray is refracted along the middle direction between zero- and (−1)-order, then the diffraction efficiency is equally split between zero- and −1st-orders and the resulted blaze height is half of the one required for 100% efficiency at (−1)-order. Still one has to go through the formal process of calculation to determine that the efficiency of (−1)- and zero-order each equals to 40.5% for the design wavelength for the corresponding diffractive lens structure and the rest of light directed along higher orders of diffraction. In terms of the terminology, one can state that light transmittance to zero and (−1) diffraction order each equals 40.5%.
Choosing the appropriate blaze spacing (rj) and blaze material thickness (hm) as set forth hereinbelow, one can produce diffractive lens of the appropriate focal length (fm) required by the ophthalmic lens application.
In a simple paraxial form the circular grating zones, also called echelettes or surface-relieve profile or grooves, can be expressed by the formula rj2=jmλf, i.e. the focal length of m-order diffraction (m=0, ±1, ±2, etc) for the design wavelength (λ) can be closely approximated by the following formula:
                              f          m                =                              r            j            2                                jm            ⁢                                                  ⁢            λ                                              (        1        )            
In the paraxial approximation the blaze material thickness to produce 100% efficiency at m-order is
                              h          m                =                              m            ⁢                                                  ⁢            λ                                (                          n              -                              n                ′                                      )                                              (        2        )            where n=refractive index of the lens material and n′=refractive index of the surrounding medium.
Diffractive lens with 100% efficiency, i.e. all light is directed along the selected diffraction order is called Kinoform lens. (J A Jordan et al. Kinoform lenses, Appl. Opt. 9, 1883–1887, (1970))
A diffractive surface may be formed by different shapes of the periodic diffractive structure and not only by specific blaze shape and for the generality of this invention the term “groove” is used as the description of the variety of shapes of the diffractive structure.
U.S. Pat. No. 5,096,285 by Silberman describes diffraction surface with 100% efficiency to provide single diffraction power and the invention does not utilize the main advantage of the diffractive optic to use several diffraction orders (zero and −1, or +1 and −1, etc.) to reduce pupil dependency of the bifocal ophthalmic lens performance.
U.S. Appl. No. 20050057720 by Morris describes also diffractive 100% efficiency surface with the utilization of multiorder diffractive surface (MOD), i.e. the zones having boundary condition of phase shift by the multiple wavelength to provide similar diffraction efficiency for the range of wavelengths instead of only for the design wavelength.
Cohen and Freeman are the principal inventors of ophthalmic multifocal diffractive optic that utilizes several diffractive orders to form image from the objects at different distances. The Cohen patents: U.S. Pat. Nos. 4,210,391; 4,338,005; 4,340,283; 4,881,805; 4,995,714; 4,995,715; 5,054,905; 5,056,908; 5,117,306; 5,120,120; 5,121,979; 5,121,980 and 5,144,483. The Freeman patents: U.S. Pat. Nos. 4,637,697; 4,641,934; 4,642,112; 4,655,565, 5,296,881 and 5,748,28 where the U.S. Pat. No. 4,637,697 references to the blaze as well as step-shapes (binary) diffractive surface.
Other patents on diffractive lenses have been granted to Futhey: U.S. Pat. Nos. 4,830,481, 4,936,666, 5,129,718 and 5,229,797; Taboury: U.S. Pat. No. 5,104,212; Isaacson: U.S. Pat. No. 5,152,788; Simpson: U.S. Pat. Nos. 5,076,684 and 5,116,111 and Fiola: U.S. Pat. Nos. 6,120,148 and 6,536,899.
Swanson in U.S. Pat. No. 5,344,447 describes tri-focal lens using binary type diffractive surface profile. Kosoburd in U.S. Pat. No. 5,760,871 also describes tri-focal lens with blaze and binary profiles.
Several patents describe the variable step size between the adjacent zones of the diffractive structure to control light transmittance at different diffraction orders with pupil size: U.S. Pat. Nos. 4,881,805 and 5,054,905 by Cohen describe so called progressive intensity bifocal lens where the step size at the adjacent zones reduced towards periphery to shift larger portion of light towards zero-order (far focus) diffraction image, i.e. to control light transmittance to the given order with pupil diameter. Baude at al in U.S. Pat. No. 5,114,220 discloses an ophthalmic lens which characteristically comprises at least two concentric regions having diffractive components with different phase profiles in order to use different orders of diffraction. Lee at al in U.S. Pat. No. 5,699,142 incorporates a similar concept into so called apodized lens by recommending the specific reduction in echelettes heights, so called apodization the diffractive surface echelettes heights, to split light initially equally between Far and Near foci (40.5% efficiency for each) and them the heights reduce towards lens periphery to shift larger portion of light towards far focus with larger pupil size, i.e. to control light transmittance with pupil diameter. Freeman in U.S. Pat. No. 5,748,282 also refers to the variable step size to control light intensity between different orders with pupil size variation.
U.S. Pat. No. 5,056,908 discloses an ophthalmic contact lens with a phase plate and a pure refractive portion within its optic zone that is placed at the periphery of phase zone area. U.S. Pat. No. 5,089,023 by Swanson also describes the lens with a combination of single focus refractive and diffractive segments that can be of bifocal design. In both inventions the refractive portion coincides with one of the diffractive order either for distant or near vision.
Thus, the diffractive optic offers the advantage to perform independently to pupil diameter. Common to all designs of the quoted patents is the fact that a bifocal diffractive lens is lacking intermediate vision. It has been shown that bifocal diffractive lens demonstrates two distinct intensities at two foci for distant and near vision (Golub M A, et al, Computer generated diffractive multi-focal lens. J. Modern Opt., 39, 1245–1251 (1992), Simpson M J. Diffractive multifocal intraocular lens image quality. Appl. Optics, 31, 3621–3626 (1992) and Fiala W and Pingitzer J. Analytical approach to diffractive multifocal lenses. Eur. Phys. J. AP 9, 227–234 (2000)). A presence of some intermediate vision reported clinically can be attributed to the aberrations of the ocular system of a given subject and not to the lens design itself.
The objective of the present invention is to provide the multifocal diffractive lens with the ability to offer a continuous focus covering far, intermediate and near vision. This would provide a naturally occurred vision similar to one through a pin-hole where a person can observe objects continually from far to near distances but without necessity to have small pupil (pin-hole) and, as a result, a very limited amount of light reaching the retina. The expectation of the lens performance according to the present invention is that the characteristic of the images of the objects at all distances from far to near are naturally occurred (pin-hole, for instance) and would inhibit a minimum of ghosting and halos commonly observed with present types of diffractive and refractive multifocal ophthalmic lenses.